Adsorption of Water as a Means to Remove Bulk Metastability of CSHS04 and RbH2P04 Bogdan Baranowski *. Milan Friesel, and Arnold Lundén Department of Physics, Chalmers University of Technology, Göteborg, Sweden Z. Naturforsch. 42 a, 565-571 (1987); received March 5, 1987 Differential scanning calorimetry (DSC) and thermogravimetry (TG) are applied to study the conditions required for CsHS04 and RbH2P04 to undergo phase transitions at 333 ± 1 K and 352 ± 3 K, respectively. It is demonstrated that water adsorption on the surface of metastable polycrystalline samples triggers their transition to the stable state. It is further shown that the transition of CsHS04 only occurs if the water vapour pressure is sufficiently high. If the sample is prepared at 293 K this threshold is 6.3 ± 0.7 mm Hg. A correlation is found between the transition temperature and the water vapour pressure. While it has been reported recently that adsorbed molecules can cause phase transitions in the surface layer of a solid matrix, our results are the first well-established examples of a direct correlation between surface adsorption and bulk phenomena. There is a relation to an early idea of Schwab. Introduction recent papers [11, 12] give no reference to the im- portance of water activity. All of these papers [10 — Some phase transitions, most of them with rather 12] are dealing with phase transitions in CsHS04. small transition enthalpies, have been discovered In paper [10] the influence of water on the phase recently in alkali metal (cesium or rubidium) hydro- transition at about 333 K was reported in rather un- gen salts (sulphates, selenates, and phosphates) [1] in clear and general terms. The authors are speaking addition to previously known similar transitions in of "moisture" on the samples, of "wet" samples, and cesium hydrogen sulphate [2], rubidium hydrogen of their "saturation" with water. They even claimed selenate [3,4] and rubidium hydrogen phosphate that solid CsHS04 "absorbs" water. In fact, they are [5, 6], The impulse for our further searching into using terms that do not have any well-defined these new transitions came from the discovery of a physical meaning and which can be misleading. surprisingly strong influence of the sample prepara- Thus, no solid hydrates of CsHS04 are known, and tion technique on the phase transitions in cesium an attempt to "saturate" with water at ambient tem- hydrogen sulphate [7], which cleared up a contro- perature would finally result in an aqueous solution. versy in the literature concerning the latter salt [2, Hence, the only realistic process to be considered is 8, 9], the adsorption of water molecules on the surface of Three main methods for the sample preparation the investigated salt. have been investigated by us: 1) grinding, 2) apply- A quantitative way to investigate the influence of ing one-dimensional (uniaxial) pressure, and 3) con- adsorbed water molecules is to maintain a known tact with water vapour of suitable pressure. The last constant water vapour pressure above the solid sam- treatment proved to be the most influential one. ples at a chosen temperature. This procedure was Similar observations are, to our knowledge, only used in our experiments. found in one recent paper [10], while two other There is a controversy between the papers [2] and [10]. The parameters given in the two papers coin- cide for the monoclinic phase of CsHS04 at room * Permanent address: Institute of Physical Chemistry, temperature (called phase III in [10]). But according Polish Academy of Sciences, ul. Kasprzaka 44/52, PL-01-224 to paper [2], no phase transition was observed dur- Warszawa, Poland. ing a determination of the heat capacity with an Reprint requests to Prof. Dr. A. Lundén, Department of Physics, Chalmers University of Technology, S-412 96 Gö- adiabatic calorimeter, which should have been the teborg/Sweden. case according to results described in paper [10]. It 0932-0784 / 87 / 0600-587 S 01.30/0. - Please order a reprint rather than making your own copy. 566 B. Baranowski et al. • Adsorption of Water and Bulk Metastability of CsHS04 and RbH2P04 is difficult to understand how the enthalpy change fluence the rate of a transition occurring in the solid accompanying the transition around 333 K (III —> II catalyser; they were not able to find any evidence in terms of paper [10]) could be lost during a heat for this in their own experiments. capacity measurement in an adiabatic calorimeter. In the most recent paper [12] the thermal history of the CSHS04 material was taken to be responsible for the observed differences between differential Experimental scanning calorimetry (DSC) peaks. According to our opinion a considerable part of these differences was As described previously [1,7], polycrystalline due to the uncontrolled amount of adsorbed water CsHS04 and RbH2P04 were prepared by crystalli- on the investigated samples. zation from aqueous solutions. The crystals were The common feature of all of the transitions crushed (without grinding), and the samples con- studied by us [1, 7] is that their characteristic DSC sisted of a loose rather coarse powder that was pre- peaks are obtained only when a properly prepared treated thermally as described below. sample is heated, while, during the subsequent cooling of the sample, the reverse transition cannot be recorded by DSC. RbH2P04 Based on these facts, the present investigation was undertaken in order to clear up two main points: The role of adsorbed water in the discussed phase transitions was tested for RbH P0 by following the 1) Either the recorded enthalpy change is due to 2 4 changes of enthalpy and weight that accompany the the formation and decomposition of a surface hy- phase transitions. The transition temperatures and drate, or the presence of adsorbed water triggers the enthalpies were determined with a commercial Ri- phase transition in the bulk phase. In the latter case gaku (Japan) DSC device. The weight changes were there should be no correlation between the mea- recorded in separate runs with a Perkin-Elmer TG-2 sured enthalpy change and any simultaneous tran- analyzer. Since it was not possible to carry out the sition of water between the gas phase and the salt. DSC and thermogravimetry (TG) simultaneously, This problem will be cleared up by studying one of each sample was divided into two parts, one for the the phase transitions of RbH P0 . 2 4 DSC and the other for the TG. 2)We wanted to see if a sample that has been in A conventional x-ray powder diffractometer contact with water vapour for a sufficient time (Philips) was used to check the nature of the 352 K undergoes a phase transition only if the water phase transition in RbH P0 . Thus, x-ray patterns vapour pressure has been above a certain value. The 2 4 taken at ambient temperature confirmed that the existence of such a threshold could be expected structure of a fresh sample was tetragonal, while it from our observation concerning the 333 K phase was monoclinic for a sample that had been subject transition of CsHS0 [7], 4 to a thermal treatment (373 K for 5 minutes); the It has recently been demonstrated for metals that first x-ray pattern was taken at atmospheric pres- adsorbed foreign molecules can cause rearrange- sure, but the second one at 10_2torr to avoid any ment of the surface molecules, i.e. two-dimensional uptake of moisture. The sample was then kept at first-order phase transitions are forced to occur ambient temperature in contact for 30 minutes with [13-16], Adsorption of hydrogen on single crystals water vapour of a pressure of 13.65 mm Hg, and the of tungsten and nickel were found to be good ex- result was that the x-ray pattern (taken at atmo- amples. In our case the adsorption of water mole- spheric pressure) had been transferred back to the cules could instead induce a three dimensional tran- original one. Thus, the salt was obtained in the sition from the metastable to the stable state of a tetragonal phase after preparation by crystallization matrix with ionic (hydrogen) bonds. This could be from aqueous solutions as well as after contact for treated as an example of a direct interaction be- some time with water vapour of sufficiently high tween surface and bulk phenomena, similar to the pressure, while cooling of an annealed sample leaves assumption by Schwab and Schwab-Agallidis [17] the salt in the monoclinic phase which is metastable that a heterogeneously catalysed gas reaction can in- at ambient temperature. 567 B. Baranowski et al. • Adsorption of Water and Bulk Metastability of CsHS04 and RbH2P04 CSHS04 Results and discussion The search for a threshold water vapour pressure RbH2P04 was performed in the following way. Crushed crys- Figure 1 shows how the pretreatment with water tals of CsHS0 were placed in open aluminium cap- 4 at ambient temperature affects the possibility of a sules of the type used for the DSC runs, and they sample of RbH P0 to undergo a structural (tetra- were heated for 1 to 4 hours in air at about 420 K. 2 4 gonal ->• monoclinic) phase transition at about 352 K. When cycled thermally, these samples showed The measured enthalpy of the incomplete transition neither the 375 K nor the 333 K transition [7]. Such of the sample depends both on the pressure of the "transition empty" samples were kept in open glass water vapour and the duration of the contact with containers surrounded by the free surface of an this water vapour. The ratio of the measured en- aqueous sulphuric acid solution within a closed, thalpy change to its maximal value (4.17 + 0.10 kJ/ thermostated volume (a small desiccator).